Optical recording method using land/groove recording

ABSTRACT

An optical reproducing apparatus for reproducing information on a medium includes a light source, an optical system for irradiating a light beam generated by the light source on the medium, a detector for detecting a reflected light from the medium; and a reproducer means for reproducing information on the medium by using a signal from the detector. The medium is a substrate having grooves and lands alternately formed thereon in a radial direction, which serve as recording tracks and have prepits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 10/938,614, filedSep. 13, 2004, now U.S. Pat. No. 7,088,652, which is a continuation ofU.S. application Ser. No. 10/463,581, filed Jun. 18, 2003 now U.S. Pat.No. 6,795,390, which is a continuation of Ser. No. 10/321,502, filedDec. 18, 2002, now U.S. Pat. No. 6,611,489, which is a continuation ofU.S. application Ser. No. 10/094,713, now U.S. Pat. No. 6,542,457, whichis a continuation of U.S. application Ser. No. 09/918,808, filed Aug. 1,2001, now U.S. Pat. No. 6,430,142, which is a continuation of U.S.application Ser. No. 09/793,889, filed Feb. 28, 2001, now U.S. Pat. No.6,314,075, which is a continuation of U.S. application Ser. No.09/394,620, filed Sep. 13, 1999, now U.S. Pat. No. 6,229,786, which is acontinuation of U.S. application Ser. No. 09/184,007, filed Nov. 2,1998, now U.S. Pat. No. 5,953,310, which is a continuation of U.S.application Ser. No. 09/059,349, now U.S. Pat. No. 5,878,008, which is adivisional of U.S. application Ser. No. 08/600,730, filed Feb. 13, 1996,now U.S. Pat. No. 5,805,565 and relates to U.S. application Ser. No.09/394,870, filed Sep. 13, 1999, now U.S. Pat. No. 6,195,316, thesubject matter of the aforementioned being incorporated by referenceherein. This application is also related to U.S. application Ser. No.09/809,048, filed Mar. 16, 2001 and U.S. application Ser. No.09/808,993, filed Mar. 16, 2001, which are continuation applications ofU.S. application Ser. No. 09/514,284, filed Feb. 28, 2000, now U.S. Pat.No. 6,262,968, which is a continuation application of U.S. applicationSer. No. 09/181,677, filed Oct. 29, 1998, now U.S. Pat. No. 6,064,644,which is a continuation application of U.S. application Ser. No.08/958,867, filed Oct. 27, 1997, now U.S. Pat. No. 5,898,663, which is acontinuation of U.S. application Ser. No. 08/733,924, filed Oct. 18,1996, which is a continuation-in-part of U.S. application Ser. No.08/600,730, filed Feb. 13, 1996, now U.S. Pat. No. 5,805,565, andapplications filed Feb. 8, 2002, the subject matter of theaforementioned applications being incorporated herein. This applicationis also related to U.S. application Ser. No. 11/176,338, filedconcurrently herewith which is also a continuation of U.S. applicationSer. No. 10/938,614, filed Sep. 13, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to optical recording methods and moreparticularly to a technique based on land/groove recording and suitablefor performing high-density optical recording in which the track widthis smaller than the optical spot diameter.

A conventional method is disclosed in, for example, JP-A-59-191156. Inthe prior art, a laser beam generated from a laser diode carried on anoptical head is formed into a collimated beam by means of a collimatinglens, and the collimated beam passing through a beam splitter is focusedby an objective lens so as to be converged into an optical spot on amagneto-optical recording medium. The position of the optical spot onthe magneto-optical recording medium is controlled by moving the lens orthe optical head by means of an optical spot scanning control means.Reflected light from the magneto-optical recording medium is guided to aphotodetector through a beam splitter. A readout signal from thephotodetector is processed by a reproduction circuit so as to beconverted into reproduced data. Control of overall reproduction iscarried out by a controller.

JP-A-6-176404 describes a technique for performing high-density (narrowtrack) recording.

A recording medium disclosed in JP-A-6-176404 is illustrated, in planview form, in FIG. 5. Grooves 501 and lands 502 are formed on asubstrate, information recording areas are formed in association withboth the groove and the land, and prepits 504 are disposed on anextension line 503 of the boundary line between a groove 501 and a land502. Prepits 504 are positioned each groove on only one side relative tothe center line of each groove. With this construction, recordinginformation is recorded on both the groove 501 and the land 502, addressinformation representative of the recording areas are recorded in theform of prepits 504, and one prepit is used in common to a pair ofadjacent groove 501 and land 502 to provide address informationtherefor.

When the technique as above is applied to, for example, a phase changerecording medium or a magneto-optical recording medium, interference ofinformation (crosstalk) between adjacent grooves 501 or lands 502 due tothe optical interference effect within an optical spot 505 can beprevented; thereby permitting narrowing of track. On the other hand, inthe prepit area free from the optical interference effect, the addressinformation can be common to the paired groove and land and theeffective track pitch can be increased to reduce crosstalk.

In the example of JP-A-6-176404, however, the disposition of the prepitarea is offset on one side of the center line of the groove and anoffset tracking error signal is delivered out of the prepit area, withthe result that when an optical spot is caused to track a groove or aland, a tracking error (tracking offset) increases, making it difficultto perform high-density recording in which the track pitch is narrowed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a technique capable ofsuppressing the tracking offset to a value which is sufficiently low forpractical use and permitting efficient disposition of addressinformation even when recording is effected on both the groove and theland.

To accomplish the above object, solutions of the invention are adoptedas below.

More particularly, in an optical recording medium having substantiallyconcentric grooves and lands formed on a circular substrate andinformation recording areas formed in association with both the grooveand the land, prepits are disposed on a virtual extension line of theboundary between a groove and a land, the disposition of the prepitssatisfying all of the following four conditions:

-   (i) Prepits are located on both sides of an extension of the center    line of one groove;-   (ii) Prepits are located on both sides of an extension of the center    line of one land;-   (iii) Prepits are not located on both sides of any specific position    of the center line of one groove; and-   (iv) Prepits are not located on both sides of any specific position    of the center line of one land.

With this construction, disposition of prepits is not offset on eitherone side of a virtual extension of the center line of the groove or theland, so that an offset tracking error signal is not delivered out ofthe prepit area, making the tracking offset hardly occur. Further, sinceprepits do not exist on both sides of or symmetrically to a position onan extension of the center line of the groove or the land, interferenceof prepit information between adjacent tracks do not take place within areproduction spot. Accordingly, recording can be performed on both thegroove and the land and addresses can be reproduced without crosstalk topermit high density narrow track recording.

Preferably, prepits are disposed alternately at a period which is eventimes a channel bit length on both sides of a virtual extension of thecenter line of the groove.

Thus, the prepits are uniformly disposed on both sides of a virtualextension of the center line of the groove or the land, making thetracking offset more hardly occur.

Further, the groove and the prepit have the same depth which is 70 nm orless. More preferably, the depth is 40 nm or more and 60 nm or less.

Through this, crosstalk between the groove and the land can duly becanceled and an excellent tracking servo signal can be obtained, thusmaking injection and production of a medium easy. With the groove depthbeing in excess of 70 nm, injection of the groove is difficult toachieve. With the groove depth being about 50 nm, tracking servo ismaximized and substantially the same effect can be obtained at a groovedepth which is 10 nm around 50 nm.

Preferably, the groove and the land have substantially the same widthwhich is in the range of from 0.3 pm to 0.75 μm.

Through this, excellent tracking can be compatible with high-densityrecording. If the groove and the land has a width which is not greaterthan 0.3 μm, two sets of groove and land are concurrently within asingle optical spot and any excellent tracking signal cannot beobtained. With the width of the groove and the land being in excess of0.75 μm, practical high-density recording cannot be permitted.

The minimal diameter of a prepit is made to be smaller than the width ofeach of the groove and land. More preferably, the diameter falls withinthe range of from 0.25 μm to 0.55 μm.

Through this, an excellent prepit signal can be obtained withoutcrosstalk. If the diameter is not greater than 0.25 μm, power of theprepit signal decreases extremely and with the diameter being in excessof 0.55 μm, crosstalk takes place.

When an optical recording medium is used in which grooves and lands areformed on a substrate, information recording areas are formed inassociation with both the groove and the land, any groove is not formedbut flat address areas are discretely formed in the informationrecording area, and first and second address pits are disposed in theaddress area on an extension of the boundary between the groove and theland, the first and second address pits being disposed to satisfy suchrequirements that the first and second address pits are disposedalternately on both sides of an extension of the center line of onegroove, that the first and second address pits are disposed alternatelyon both sides of an extension of the center line of one land, thataddress pits do not exist on both sides of a position on an extension ofthe center line of the groove and that address pits do not exist on bothsides of a position on an extension of the center line of the land, anoptical spot is irradiated on the optical recording medium, a reflectedbeam from the optical recording medium is detected, an address pit isdetected from the detected reflected beam to form an address pit readoutsignal, an address is detected on the basis of the address pit readoutsignal, an amplitude of a first readout signal obtained from the firstaddress pit of the address pit readout signal is sampled and held, anamplitude of a second readout signal obtained from the second addresspit of the address pit readout signal is sampled and held, theamplitudes of the first and second readout signals are comparedtogether, an offset signal is formed on the basis of a result ofcomparison, and the irradiation position of the optical spot iscontrolled on the basis of the offset signal.

Through this, the tracking offset can be suppressed sufficiently forpractical use and address information can be obtained.

When tracking is carried out by sequentially obtaining tracking servosignals through the use of a diffracted beam obtained from a groove andcorrecting an offset of a tracking servo signal with an offset signal,stabler tracking can be ensured. More particularly, upon detection of areflected beam from the optical recording medium, a tracking servosignal is formed by detecting a light beam diffracted by a groove bymeans of a plurality of photodetectors, comparing diffracted beamsdetected by the plurality of photodetectors and detecting the relativepositional relation between the groove and the optical spot, thetracking servo signal is corrected with an offset signal, and theirradiation position of the optical spot is controlled on the basis ofthe corrected tracking servo signal.

Further, an optical recording/reproducing apparatus may be constructedwhich uses a similar optical recording medium, comprises a light beamsource, a beam focusing means for focusing and irradiating a light beamgenerated by the light beam source on the optical recording medium,photodetecter detecting a reflected beam of the light beam irradiated bythe beam focusing means, a reproduction circuit reproducing informationby using a signal from the photodetector, and a scanner moving theposition of an optical spot irradiated by the beam focusing means to adesired position on the optical recording medium, and further comprisesmeans for detecting an address on the basis of a readout signal from aprepit, a low-pass filter for detecting an amplitude of a low frequencycomponent of the readout signal from the prepit, and a circuitperforming lock-in detection of a passed signal of the low-pass filter,whereby the position of the optical spot is controlled on the basis ofthe detected signal.

With this construction, the tracking offset can be suppressed to asmaller value.

According to an aspect of the present invention, as shown in for exampleFIG. 1, prepits are disposed on both sides of a virtual extension of thecenter line of the groove or the land in staggered relation.

Accordingly, offset can be decreased to make the tracking offset hardlyoccur and prepits do not exist on both sides of a position on theextension of the center line of the groove or the land, with the resultthat interference of prepit information between adjacent tracks can beprevented within a reproduction spot and high-density narrow trackrecording can be ensured.

Further, even if a tracking offset takes place as shown in FIG. 3,signal amplitudes of prepits on both sides are compared to performaccurate detection of the tracking offset amount. Accordingly, byfeedback-controlling the information to the scanner, the tracking offsetcan be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an embodiment of an optical recording mediumaccording to the present invention.

FIG. 2 is a fragmentary plan view of the FIG. 1 optical recordingmedium.

FIG. 3 is a waveform diagram showing readout waveforms in the presentinvention.

FIG. 4 is a block diagram of an embodiment of an opticalrecording/reproducing apparatus according to the present invention.

FIG. 5 is a fragmentary plan view of a prior-art optical recordingmedium.

FIG. 6 is a fragmentary plan view of another embodiment of the opticalrecording medium according to the present invention.

FIG. 7 is a waveform diagram showing readout waveforms obtained from theFIG. 6 optical recording medium.

FIG. 8 is a block diagram of an embodiment of an optical recordingapparatus according to the present invention.

FIG. 9 is a waveform diagram useful to explain the principle of theoptical recording apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is illustrated, in plan view form, anoptical recording medium of the present invention. Grooves 84 eachhaving a width of 0.6 μm and a depth of 50 nm and lands 85 each having awidth of 0.6 μm are formed alternately in the radial direction of themedium and recording marks 81 are recorded on the two kinds of areas.More particularly, the land 85 and the groove 84 are both recordingareas which form tracks. In a prepit area 83, any groove is not formedbut prepits 82 are disposed on a flat area serving as an address area.This type of optical recording medium can be produced by forming arecording film on a disk-like substrate having grooves 84 and prepits82, which substrate being prepared through mass producing using astamper. The prepit area 83 is formed radially of the substrate, havingindicia such as marks in the form of prepits 82 which are disposedconcentrically across a plurality of tracks. The prepits are not alwaysrequired to be arranged in the radial direction over the entire radiusof the substrate and the prepit area may be divided into a plurality ofzones which are arranged in the radial direction, forming a ZCAV (zonedconstant angular velocity) or ZCLV (zoned constant linear velocity) typeformat which is preferable from the standpoint of high densityrecording.

Referring to FIG. 2, the neighborhood of the prepit area 83 isillustrated in an enlarged view form. Pits 82 are disposed on anextension line of the boundary between a land and a groove. A pit has awidth of 0.35 μm and a depth of 50 nm. The prepit area is divided into afirst prepit area 831 and a second prepit area 832. In the first prepitarea 831, pits 82 are disposed on the upper side, in the drawing, of thecenter line of a land 85 but in the second prepit area 832, pits 82 aredisposed on the lower side, in the drawing, of the center line of theland 85. Accordingly, when an optical spot 21 scans, for example, theland 85, a signal is always produced from either one of the first andsecond prepit areas and consequently, there is no fear that crosstalkwill occur between adjacent tracks. Therefore, address informationrecorded in the form of prepits can duly be reproduced withoutcrosstalk.

Since pits 82 are not adjoined in the radius direction, injection can befacilitated upon formation by the stamper.

Also, pits 82 are uniformly disposed on both sides of a track (a land ora groove) and hence the influence of a tracking error signal, which isdelivered out of the prepit area while being offset due to pits 82, upona tracking servo signal can be canceled. Accordingly, the trackingoffset can be suppressed to a minimum.

Further, when reproducing, for example, a land 85, reproduction ofaddress information at the second prepit area 832 is carried outcontinuously with reproduction of address information at the firstprepit area 831.

Accordingly, when the two areas are united into one area in whichinformation is arranged to provide address information for one track, anaddress (track number) of a land and that of a groove can be setindependently of each other.

In the present embodiment, a magneto-optical recording film (TbFeCo) isused as the recording film. Accordingly, the recording mark is preparedin the form of a recorded domain. A known phase change film may also beused as the recording film. In the example of FIG. 2, one set of firstand second prepit areas is provided but a plurality of sets may beprovided as necessary.

Referring to FIG. 6, there is illustrated, in enlarged fragmentary planview form, another example of the optical recording medium of thepresent invention. Grooves 84 each having a width of 0.5 μm and a depthof 40 nm and lands 85 each having a width of 0.5 μm are formedalternately and recording marks 81 are recorded—on the two kinds ofareas. Thus, the land 85 and the groove 84 are both recording areas. Ina prepit area 83, any groove is not formed but substantially circularpits 82 (each having a diameter of 0.3 μm and a depth of 40 nm) aredisposed on an extension line of the boundary between a land and agroove. The prepit area is divided into a VFO (Variable FrequencyOscillator) area 833 and an address area 834.

Especially, in the VFO area, pits 82 are disposed alternately on theupper and lower sides of the center line of a land 85. In the addressarea, pits 82 are disposed alternately at the same period as that in theVFO area. Accordingly, there are no pits which exist on both sides of(or symmetrically to) a position on the center line of the land and thegroove. In addition, in the address area, data for a particular track isso encoded as to differ by one pit from data for an adjacent track. Inother words, the data takes the form of a Gray code. With thisconstruction, when an optical spot 21 scans, for example, a land 85,pits on either one side are always reproduced and there is no fear thatcrosstalk will occur between adjacent tacks. Therefore, addressinformation distributed to the prepits can duly be reproduced withoutcrosstalk. Since pits 82 for adjacent tracks do not adjoin to eachother, injection can be facilitated. Also, pits 82 are uniformlydisposed on both sides of a track (a land or a groove) and hence theinfluence of a tracking error signal, which is delivered out of theprepit area while being offset due to pits 82, upon a tracking servosignal can be canceled. Accordingly, the tracking offset can besuppressed to a minimum.

Referring to FIG. 7, readout signals obtained from the prepit area 83 inthe FIG. 6 embodiment are illustrated. When an optical spot scans thecenter of individual tracks, signal waveforms shown in the Figure aregenerated of which signals 11 are generated from tracks constructed oflands 85 and signals 12 are generated from tracks constructed of grooves84. As is clear from the Figure, the generated signals are different forthe individual tracks, demonstrating that address information isrecorded very efficiently. By virtue of the use of the Gray code, anaddress can be reproduced in the course of inter-track access, ensuringsuitability to high-speed access. Further, the use of the Gray codemakes an error hardly occur even in the presence of crosstalk, thusensuring suitability to narrowing of tracks.

Referring now to FIG. 4, there is illustrated an example of constructionof an optical recording/reproducing apparatus of the present invention.

In the present embodiment, a laser diode 311 having a wavelength of 680nm is used as a light source, a laser beam is formed into a collimatedbeam by means of a collimating lens 312, and the laser beam is focusedto an optical spot 21 on an optical disk 8 by means of an objective lens321. As necessary, a beam shaping means such as a prism and other lensesmay be provided in the optical path. The optical disk shown in FIGS. 1and 2 is used as the optical disk 8.

Power of the laser diode 311 is controlled by a light power controller71 having the auto-power control function. Beam splitters 324 and 325are adapted to guide a light beam 23 reflected from the optical disk 8to photodetectors 333, 334, 340 and 341. In the present embodiment, theaperture ratio of the objective lens 321 is set to 0.55. Consequently,the diameter of the optical spot 21 on the optical disk 8 is 1.1 μm.

The optical disk 8 is rotated by a motor 62. The optical spot 21 can bemoved to a desired position on the optical disk 8 by means of a scanningmechanism. In the present embodiment, the scanning mechanism, asdesignated at 6, has an automatic position controller 6 also designatedat 6 and having functions of auto-focus control and automatic tracking,and a lens actuator 61 controlled by the automatic position controller6.

The reflected beam 23 from the optical disk is guided to a signaldetection system by means of the beam splitters 324 and 325. Part of thereflected beam is split to two beams having different polarizationplanes through a half-wave plate 337, a lens 331 and a polarized beamsplitter 332 and the two beams are detected by the photodetectors 333and 334, respectively. Readout signals from the two detectors 333 and334 are differentially amplified by a differential amplifier 944 so thatinformation magnetically recorded on the optical disk may be detectedmagneto-optically. The readout signals of the two detectors 333 and 334are also added together by means of an adder 941 so that information 14recorded in the form of prepits on the optical disk may be detected.

The automatic position controller 6 utilizes the reflected beam 23 fromthe optical disk 8 to cause a tracking servo signal detector 34 todetect an optical spot position which is used for feedback control. Fordetection of the optical spot position, the detectors 340 and 341 detectpower of a diffracted light ray from a groove in the optical disk 8 andsignals delivered out of the detectors 340 and 341 are differentiallyamplified by a differential amplifier 342 to produce a differencesignal.

Illustrated in FIG. 3 are signals obtained from the optical disk 8 shownin FIGS. 1 and 2 in the present embodiment. When the optical spot 21scans a land 85, a signal wave including magneto-electrical reproducedsignal 12 shown in under side of FIG. 3 is obtained. As shown in FIG. 3,when the optical spot 21 deviates from the track center (being offset),an amplitude difference 13 takes place between prepit signal portionsfrom the first and second prepit areas 831 and 832. This amplitudedifference 13 corresponds to an amount of tracking offset.

The prepit signal 14 shown in FIG. 3 is fed to an address detectionmeans 43 so as to be decoded to address information by an addressdecoder 431. At the same time, timings for signals of the first andsecond prepit areas are generated by a timing controller 432.

On the basis of the timing information, a first amplitude sample andhold circuit 411 stores an amplitude (average maximum amplitude) of thefirst prepit area and a second amplitude sample and hold circuit 412stores an amplitude (average maximum amplitude) of the second prepitarea. Alternatively, the first and second amplitude sample and holdcircuits 411 and 412 may have a common amplitude sampler.

The thus held amplitudes are compared together by means of an amplitudecomparator 42 to produce an amplitude difference 13. On the basis of theamplitude difference 13, a tracking offset signal 44 is formed. Thetracking offset signal 44 is added with a tracking error signal from theservo signal detector 34 by means of an adder 942 to produce a sumsignal which in turn is fed back to the position moving means (scanningmeans) 6.

In the apparatus of the present embodiment, the tracking offset signalis formed on the basis of the amplitude difference 13 and the trackingerror signal is corrected with the tracking offset signal to produce acorrected signal which is fed back to the position moving means.Accordingly, even when various kinds of external disturbance such asaberration of the optical spot is taken into consideration, the trackingoffset can be decreased to ±0.03 μm or less. Under the nominal statedevoid of optical aberration, the tracking offset is ±0.015 μm or less.

In carrying out recording with the apparatus of the present embodiment,a recording beam 22 whose power is controlled by the light powercontroller 71 is irradiated on the optical disk 8 to form an opticalspot 21. While applying a bias field to the neighborhood of the opticalspot 21 by means of a bias field power controller 72, a bias fieldapplication circuit 73 and a bias coil 74, the temperature of therecording film is heated by the optical spot 21 to a value near theCurie temperature to form a recorded domain in a heated area. In thisexample, the size of the recorded domain is assumed to be of a width ofabout 0.5 μm.

Referring to FIG. 8, there is illustrated another example ofconstruction of the optical recording/reproducing apparatus of thepresent invention. This example differs from the FIG. 4 embodiment inthat the prepit signal is passed through a low-pass filter 45 andlock-in amplified by a lock-in amplifier (detector) 44, thereby formingtracking error information. In this example, the optical recordingmedium of FIG. 6 is used and therefore, pits 82 are disposed alternatelyon both sides of the center of a land 85 or a groove 84 in the VFO area833.

Illustrated in FIG. 9 are waveforms of a VFO signal (output of theamplifier 941) and a low-pass filter signal (output-of the LPF 45). Inthe absence of an offset, a low-pass filter component 16 of a VFO signalhas no amplitude but in the presence of a tracking offset, a low-passfilter component 18 of a VFO signal 17 has an amplitude. This amplitudeis lock-in amplified by the lock-in amplifier 44 to detect a trackingoffset. Accordingly, by feedback-controlling the offset amount to thescanning means 6, the tracking offset can be decreased. In the presentembodiment, the tracking offset can be suppressed to ±0.025 μm or less.

The present invention is in no way limited to the foregoing embodiments:For example, an optical head capable of generating a plurality ofoptical spots at a time may also be used. In addition to themagneto-optical recording medium, a phase change recording medium may beused. Further, in addition to the method using the diffracted beam, athree-spot detection method in which power levels of reflected beamsfrom a plurality of optical spots are compared together or apre-wobbling method may be used as the servo signal detection method.

By using the optical recording medium of the present invention, thetracking offset can be suppressed to a level which is sufficiently smallfor practical use (0.03 μm or less) and address information can beobtained easily even during high-density narrow track recording. Byusing the optical recording/reproducing apparatus of the presentinvention, the tracking offset can be decreased easily through feedbackcontrol.

1. An optical reproducing apparatus for reproducing information on amedium comprising: a light source; an optical system for irradiating alight beam generated by the light source on the medium, the opticalsystem including an automatic position controller enabling auto-focuscontrol and automatic tracking control for controlling a lens actuator;a detector for detecting a reflected light from the medium; andreproduction means for reproducing information on the medium by using asignal from the detector; wherein the medium comprises: a substrate,grooves and lands alternately formed on the substrate in a radialdirection, the grooves and the lands both serving as recording tracks,the recording tracks having information recorded thereon and beingdivided into recording units in a circumferential direction, each therecording units having a prepit area in a non-groove portion of thesubstrate, a first prepit representative of VFO information beingprovided in the prepit area, and a second prepit representative ofaddress information being provided in the prepit area, the first prepitand the second prepit being located on both sides of a center line ofone track, the first prepit and the second prepit being formed on eachside of the center line of the one track so as to be shared with anadjacent track which is adjacent each side of the center line of the onetrack, and every prepit including the first prepit and the second prepitnot existing at opposing positions on both sides of the center line ofthe one track.